Block and tackle window balances combine a system of pulleys with an extension spring to convert high spring tension applied over a short working distance to a lower spring tension applied over a longer working distance. The extension spring and pulley system are arranged within a rigid channel, with the extension spring anchored at one end and the pulley system anchored at the other end. The balance channel is attached to a window sash or a window frame; and a cord, which is reeved through the pulley system, is attached to the other of the window sash or frame. The extension spring and pulley system are sized so that a desired lifting force is applied to the window sash throughout the entire range of sash travel within the window frame.
Special couplers are used to connect a free end of the extension spring to the pulley system. For example, commonly assigned U.S. Pat. No. 4,672,713 to Newton et al. discloses a tube-shaped coupler projecting from a movable block of a pulley system. The coupler has a recess that receives an indented coil at the free end of an extension spring to provide an interlock between the coupler and the extension spring.
Another commonly assigned patent, namely, U.S. Pat. No. 4,689,850 to Flight, discloses a coupler having a similar tube-shaped section for engaging an extension spring. However, Flight's coupler also has a plate-shaped section for mounting an axle of a movable pulley block and a hook formed at one end of the plate-shaped section for attaching one end of a pulley cord. The axle is formed by a pair of interlocking rivets having journals for supporting respective pulleys and heads for trapping the respective pulleys against opposite sides of the plate-shaped section.
Couplers having tube and plate sections are difficult to make to required accuracy and add cost to window balances. Accordingly, the tube sections have been replaced by extending the plate sections to include triangular heads with two abutment surfaces for engaging tapered ends of the extension springs. Coils on the free end of extension springs are crimped tightly around a neck portion of the entirely plate-shaped couplers to interlock with the abutment surfaces.
Although the plate-shaped couplers are easy to manufacture at low cost, difficulties persist with maintaining proper alignment of the extension springs within the balance channel. Small inaccuracies in the crimping of the extension springs to the plate-shaped couplers can cause the extension springs to scrub against the walls of the channel, creating noise and additional friction that can cause the cords to fail from extra strain. Persistent scrubbing can also cause a failure of the channel walls.
Prior block and tackle balances, such as the balance system disclosed in the coassigned patent to Flight, include relatively expensive arrays of hardware for anchoring fixed pulley blocks to channel ends. The fixed pulley blocks include two sets of pulleys having axles mounted in tandem along support plates. A series of bends are made in the support plates to form hooks for attaching the support plates to the channel ends.